Advances in plasma processing have facilitated growth in the semiconductor industry. During plasma processing, diagnostic tools may be employed to ensure high yield of devices being processed. Optical emission spectroscopy (OES) is often utilized as a diagnostic tool for gas-phase monitoring of etchants and etched products to maintain tight control of process parameters.
In the optical interrogation of plasma, there is a characteristic glow, i.e., specific optical emission spectrum, associated with a plasma discharge. With spectral information from the optical interrogation, a considerable amount of information on constituent species may be collected and analyzed to provide guidance for process monitoring and control during plasma processing.
To facilitate discussion, FIG. 1A shows a simplified schematic of a prior art plasma processing chamber 100 with an optical viewport, i.e., window, 102 that has a direct line of sight to plasma 104. As the term is employed herein, a line of sight is an optical straight-line path from a source to a collector without any form of obstruction.
Consider the situation wherein, for example, optical interrogation of plasma 104 is being performed. Due to the optical path length, the line of sight may have some arbitrary angle of acceptance at optical viewport 102. As the term is employed herein, angle of acceptance is an angle from the distal end of viewport 102 which a non-axial light source may still reach the collector end of optical viewport 102.
In an example, to perform endpoint detection via optical interrogation, it may be desirable to collect a signal source from a specific area of plasma 104 with a particular line of sight at viewport 102. The area outside of the angle of acceptance may include diffraction and/or reflection of light from other surfaces in plasma processing chamber 100. Thus, a low angle of acceptance may be highly desired for optical interrogation of the specific area of plasma 104. However, the simple setup of viewport 102 in the example of FIG. 1A may lend itself to a larger angle of acceptance.
In order to examine the spectra emission of the plasma, optical viewport 102 may be required to be optically transparent to the measured wavelength(s). For infrared (IR) to ultraviolet (UV) wavelengths, optical viewport 102 may be constructed from some type of fused silica, e.g. semiconductor grade or UV optical grade. One of the issues with placing the optically transparent viewport within a region close to the plasma is the potential to get deposition or etching on optical viewport 102 over time.
In general, optically transparent windows may be clouded or be eroded during plasma processing. When optical signal degradation from these causes reaches a level that impacts system performance, equipment must be removed from service to regain initial functionality. System downtime increases maintenance costs (removal of parts, cleaning or replacement, reinstallation) and reduces overall production output.
Consider the situation wherein, for example, a fabrication process is fine-timed to a particular set of quantitative values from a baseline process at time zero using optical interrogation. During plasma processing, the fabrication process compares experimental values with baseline values to deliver optimal yield. However, if optical viewport 102 has been compromised by deposition or etching over time, the fabrication process may not be able to determine if the drift in transmission of signal intensity is due to the change in plasma properties or due to viewport 102 being conditioned.
Furthermore, the change in transmission of signal intensity may be wavelength dependent. For example, the signal intensity of UV to IR spectrum is being examined for the plasma process. At time zero, the UV to IR spectrum may transmit at 100 percent. However, at time X UV wavelengths may transmit at 50 percent and IR wavelengths may transmit at 90 percent. Thus, it may be difficult to quantitatively separate changes in the plasma spectral signal from changes in window transmittance over some finite time period.
FIG. 1B shows a simplified schematic of a prior art solution employing a collimator 128 in a plasma processing chamber 120. In the example of FIG. 1B, collimator 128 is coupled to an optical viewport 122 with a direct line of sight to plasma 124. In an example, collimator 128 has an arbitrary length (L) and diameter (D). The dimensionless ratio of L over D, i.e., (L/D), is the aspect ratio.
By employing collimator 128, the example of FIG. 1B may have been able to address some of the issues of viewport conditioning and/or angle of acceptance of non-axial light. For example, the higher the aspect ratio (L/D) of the collimator the larger the value of L is compared to D. In the case of angle of acceptance, the amount of non-axial light reaching the collector end of collimator 128 is minimized as the aspect ratio increases. Analogously, due to molecular diffusivity, the quantity of materials from reaction chamber reaching optical viewport 122 is also minimized. Therefore, a collimator with high aspect ratio is very desirable due to the aforementioned benefits.
FIG. 1C shows a simplified schematic of a prior art solution with a long collimator, 148, in a plasma processing chamber 140. In the example of FIG. 1C, collimator 148 is coupled to an optical viewport 142 with a direct line of sight to plasma 144. In an example, collimator 148 has a length (L) and diameter (D). To attain a high aspect ratio, the value of L is larger than the value of D in the example of FIG. 1C.
In order to minimize deposition or etching on viewport 142, an aspect ratio of greater than 10:1 is often needed. Analogously, in order to get to a small angle of acceptance, e.g., less than 2° line of sight for suitable optic coupling, an aspect ratio upward of about 30:1 is desired. Consider the situation wherein, for example, collimator 148 has a diameter (D) of about 1 inch. To minimize viewport conditioning and/or achieve small angle of acceptance, collimator 148 may need to be 30 inches in length (L) to attain an approximately 2° acceptance angle. However, a collimator of 30″ in length is not, at this time, a practical solution for plasma processing equipment.
FIG. 2A shows a simplified schematic of a prior art solution of a collimator 228 with a single, small diameter hole in a plasma processing chamber 220. In the example of FIG. 2A, collimator 228 is coupled to an optical viewport 222 with a direct line of sight to plasma 224.
To create a compact collimator with high aspect ratio, the diameter of the hole in the collimator may be reduced. For example, collimator 228 is a solid tube with a hole 230 of diameter with a first pre-determined value bored through the length of the tube. The diameter of the hole with the first pre-determined value for collimator 228 of FIG. 2A is smaller than the diameter of the hole with a second pre-determined value for collimator 148 of FIG. 1B.
To reduce the length of the collimator aid still maintain a high aspect ratio, a collimator may have a hole of diameter that is 1/10 of the size of 1 inch for example. The length of collimator 228 would therefore be 1/10 of the collimator length required for a hole diameter of 1″ to obtain a specified aspect ratio. However, a collimator with a very small diameter hole may not be practical since the amount of light being collected is minimal. Therefore, the ability to use most spectrophotometers to process the signal is impractical due to unreasonable integration time and/or high signal to noise ratio.
FIG. 28 shows a simplified schematic of a multi-holed collimator in plasma processing chamber 240. In the example of FIG. 2A, collimator 248 is coupled to an optical viewport 242 with a direct line of sight to plasma 244. Collimator 248 is a solid tube with a plurality of holes (250A . . . 250N) of diameter (D) bored through the length of the tube.
Collimator 248 of FIG. 2B may be configured with a full array of holes to collect upwards of 60% of light incident to the whole area of the distal end of collimator 248. From the manufacturability standpoint, collimator setup of FIG. 2B may be easily machineable to an aspect ratio of about 10:1. However, beyond the 10:1 aspect ratio it may become very difficult and expensive to machine holes that retain a good degree of perpendicularity to the collection area. Although an aspect ratio of 10:1 may reduce optical viewport 242 conditioning, a higher aspect ratio, i.e., 30:1, is required to attain a small angle of acceptance for direct line of sight to minimize non-axial light collection.
Unfortunately, the aforementioned prior art collimators may not provide the optimal solution to optical interrogation of plasma employing optical emission spectroscopy, laser induced fluorescence, particle detection, optical absorption spectroscopy, or other optical interrogation methods used in semiconductor processing environments. To overcome the problems of deposition or etching on optical viewport and/or high angle of acceptance, collimators with high aspect ratio are desirable. To attain the required high aspect ratio, prior art solutions may not be practical, too expensive, or outside current machining capability.